1. Field of the Invention
The present invention relates to a method for manufacturing a MOS transistor, and more particularly to a method for manufacturing a MOS transistor having excellent electrical characteristics.
2. Description of the Related Art
For a semiconductor substrate for a LSI circuit with high density integration of MOS transistors, a silicon semiconductor substrate is generally used. A possible way to improve the operating speed of this LSI circuit is to increase the electrostatic capacity between a gate formed on a gate oxide film in the active region of the semiconductor substrate for the MOS transistor and the active region below the gate.
This electrostatic capacity is connected with the thickness of the gate oxide film and the dielectric constant of the material of the gate oxide film. To take an example, by using tantalum oxide (Ta.sub.2 O.sub.5) having a high specific dielectric constant of about 25 to form the gate oxide film, the electrostatic capacity can be increased without making the gate oxide film so thin as to cause a problem of stray current due to the thinning of the gate oxide film.
Meanwhile, the gate oxide film consisting of tantalum oxide is formed in an atmosphere of oxygen gas. For this reason, a silicon oxide film (SiO.sub.2) with a low dielectric constant is formed between the tantalum oxide and the silicon semiconductor substrate, and this silicon oxide film impedes the increase of the electrostatic capacity.
Therefore, it has been proposed to adopt a multi-layered gate oxide film consisting of a silicon nitride layer and a tantalum oxide layer, made by first forming on the active region a silicon nitride layer (Si.sub.3 N.sub.4) that shows a higher dielectric constant than that of the oxide film and inhibits the growth of the silicon oxide and then forming a tantalum oxide layer on the silicon nitride layer (e.g., JP-A-07-167008).
As the active region is covered with the silicon nitride layer before the tantalum oxide layer is formed on the active region of the silicon substrate, the silicon nitride layer prevents oxygen from penetrating into the silicon substrate during the formation of tantalum oxide layer on the silicon nitride layer. Therefore, the growth of a silicon oxide layer is inhibited, making it possible to prevent a decrease in the electrostatic capacity due to the growth of the silicon oxide layer and thus secure a desired electrostatic capacity.
As technique for forming a silicon nitride layer on a silicon semiconductor substrate as mentioned above, there are methods such as RTN (Rapid Thermal Nitrization) for directly nitrizing a silicon semiconductor substrate in an ammonia gas atmosphere (JP-A-05-167008), LPCVD (low-pressure CVD, JP-A-04-269859), Jet Vapor Deposition (by S. Mahapatra et al., 1999 VLSI Tech. Dig. P.79) and a nitrizing method using a high density plasma (Katsuyuki Sekine et al. 1999 VLSI Tech. Dig. P.115).
However, any of those methods uses ammonia (NH.sub.3) as a nitrizing gas and by heat treatment in an ammonia gas atmosphere, hydroxyl groups (OH) produced by combinations of oxygen atoms from a naturally oxidized film on the semiconductor substrate with hydrogen atoms from the ammonia are introduced into the gate oxide film. The OH groups in the gate oxide film act as a charge trap, resulting in deterioration of the electrical characteristics of the MOS transistor.
Therefore, the object of the present invention is to provide a method for manufacturing a MOS transistor capable of improving its operation speed by increasing the electrostatic capacity without incurring the deterioration of electrical characteristics caused by a charge trap.